Photographic processes

ABSTRACT

SILVER HALIDE GRAINS, HAVING CERTAIN METHINE SENSITIZING DYES ADSORBED THERETO, ARE RENDERED SENSITIVE TO INFRARED RADIATION LONGER THAN ABOUT 2.0 MICRON BY COOLING THE GRAINS TO BELOW ABOUT -180*C., AND UNIFORMLY EXPOSING THE COOLED GRAINS TO RADIATION HAVING A WAVELENGTH FROM ABOUT 370 TO ABOUT 460 NM. TO PRODUCE A DEVELOPABLE LATENT IMAGE. AN IMAGEWISE INFRARED HERSCHEL EXPOSURE OF THE COOLED GRAINS CAUSES IMAGEWISE DESTRUCTION OF THE DEVELOPABLE LATENT IMAGE.

United States Patent @fice 3,685,997 PHOTOGRAPHIC PROCESSES Lillian M.Kellogg and Nancy B. Liebert, Rochester, N.Y., assignors to EastmanKodak (10., Rochester,

NZ) lJrawing. Filed Apr. 21, 1971, Ser. No. 136,172 Int. (:1. G03c 1/28,5/32 U.S. c1. 96--107 8 Claims ABSTRACT OF THE DISCLOSURE Silver halidegrains, having certain methine sensitizing dyes adsorbed thereto, arerendered sensitive to infrared radiation longer than about 2.0 micron bycooling the grains to below about 180 C., and uniformly exposing thecooled grains to radiation having a wavelength from about 370 to about460 nm. to produce a developable latent image. An imagewise infraredHerschel exposure of the cooled grains causes imagewise destruction ofthe developable latent image.

This invention relates to photographic processes and more particularlyto photographic processes in which silver halide emulsions are exposedat low temperatures.

James, in US. Pat. application 822,670 filed May 7, 1969, and inPhotographic Science and Engineering, vol. 14, No. 1, January-February1971, pp. 84-86, shows that the sensitivity and contrast of photographicsilver halide emulsions to be exposed at low temperatures (i.e., belowabout -180 C.) can be increaed by adsorbing certain methine dyes to thesilver halide grains. The useful methine dyes described by James arethose in which the highest occupied electronic energy level of theunaggregated dye at room temperature is more positive than the highestoccupied energy level of the valence band of the silver halide, and thelowest vacant electronic energy level of the dye, in its unaggregatedform at room temperature, is more positive than the conduction band ofthe silver halide. The dyes are used in relatively large concentrationswith best results being obtained when the grains have a monolayercoverage of the dye.

James et al. in Photographic Science and Engineering, vol. 4, No. 4(1963), describe the use of the Herschel effect on a low temperaturesilver halide emulsion as a means for detecting infrared radiation up toabout 1.8 microns, and suggests there might be some utility in carryingout the process to detect LR. radiation of up to 2.0 microns inwavelength. No disclosure is known in the art for efifectivelyincreasing the sensitivity of a low temperature silver halide emulsionto infrared radiation of greater than 2.0 microns.

Accordingly, this invention is directed toward photographic processes inwhich silver halide photographic emulsions are exposed at lowtemperatures. In one aspect the present invention is directed tophotographic processes producing a silver halide emulsion havingincreased infrared sensitivity when exposed at low temperatures on theorder of about -l80 C. Still another aspect of the present invention isto provide photographic processes useful for detecting long wavelengthinfrared radiation, e.g., infrared radiation having wavelengths from 2.0to about 3.5 microns. Other aspects of the invention will be apparentfrom the disclosure herein and the appended claims.

In accordance with this invention, a photographic process is providedfor increasing the infrared sensitivity of silver halide grainsmaintained at a temperature below about 180 C., said grains havingadsorbed thereto a methine sensitizing dye, the highest occupiedelectronic energy level of said dye, in its unaggregated form at roomtemperature, being more positive than the highest occupied energy levelin the valence band of the silver halide, and

Patented Aug. 22, 1972 the lowest vacant electronic energy level of saiddye, in its unaggregated form at room temperature, being more positivethan the conduction band of the silver halide, said dye trappingelectrons at a depth of not more than .7 ev. below the conduction bandof the silver halide at temperatures below about 180 C., the silverhalide grains having a sufficient amount of the dye adsorbed thereto toeifectively increase the speed of the silver halide when exposed at atemperature below about l C., which comprises:

(1) cooling the photographic silver halide emulsion to a temperaturebelow about C.; and,

(2) uniformly exposing the cooled emulsion to blue light of a wavelengthof from about 370 to about 460 nm. for sufiicient time to produce alatent image having a developable density.

Preferably, the overall low temperature exposure to radiation of about370 to about 460 nm. is sufficient to produce a developable density ofat least 1.0 or higher. The latent image produced by the overallexposure can be removed by a long wavelength infrared imagewise exposurewhile the emulsion is maintained at the low temperature. The emulsion isnot warmed to a temperature substantially above about 180 C. between theoverall exposure and the imagewise infrared exposure.

The low temperature infrared exposure destroys the latent image producedby the low temperature overall exposure, the amount of destruction ofthe latent image varying with the intensity and wavelength of theinfrared radiation striking the emulsion. In view of the James et al.disclosure, it was very surprising that the present process rendersemulsions sensitive to infrared radiation extending well beyond the 2.0micron limit.

Any methine dye can be used in the practice of the invention, providedthe highest occupied electronic energy level of the dye adsorbed on thesilver halide, in its unaggregated form at room temperature, is morepositive than the highest occupied energy level in the valence band ofthe silver halide, and the lowest vacant electronic energy level of thedye adsorbed on the silver halide is, in its unaggregated form at roomtemperature, more positive than the conduction band of the silverhalide. The highest occupied electronic energy level of methine dyesadsorbed on silver halide is preferably more negative than theconduction band of the silver halide. The highest occupied electronicenergy level of a methine dye adsorbed on silver halide, and the lowestvacant electronic energy level of a methine dye adsorbed on silverhalide, can be calculated by the method described by Tani and Kikuchi,Photographic Science and Engineering, vol. 11, No. 3, p. 129 (1967) andTani, Kikuchi and Honda in Photographic Science and Engineering, vol. 2,p. 80 (1968).

At temperatures below about 180 C., the dyes employed herein trapelectrons at a depth of not more than .7 ev., and preferably not morethan .4 ev., below the conduction band of the silver halide. Thesecalculations can be made by the procedure described by Berry in TheJournal of Photographic Science, 1970, vol. 18, No. 5, pp. 169 and 174.

The highest occupied energy level in the valence band of silver halideand the bottom conduction band of the silver halide has been determinedby those skilled in the art. See, for example, Mees and James, TheTheory of the Photographic Process, third edition, (the MacMillanCompany, 1966), pp. 19-21, 264 and 265, and Tani, Kiknchi and Honda,Photographic Science and Engineering, volume 12, No. 2 (196 8) p. 80.The highest occupied energy level of the valence band of silver halideis about 6.0 ev. relative to 0 for vacuum, and the bottom of theconduction band of silver halide is about 3.5 ev. The values which havebeen calculated for pure silver bromide result in an energy level in thevalence band of approximately 6.0 ev. relative to for vacuum, with theelectronic energy of the bottom of the conduction band of the silverbromide being approximately --3.5 ev. Further description of themechanism for calculating the highest occupied energy level in thevalence band and bottom conduction band of silver halides is given inGrundlagen der photographichen Prozesse mit Silberhalogen-idem; Band 3,1968, Akademische Verlagsgesellschaft, pages 1068-1071.

Any of the methine dyes, including the styryl and cyanine methine dyes,can be employed in the practice of the invention, provided the dyes meetthe criteria given above. Especially good results are obtained withmethine dyes which meet the above criteria and have at least one of thefollowing formulas:

In the above formulas, h, m, n and p each represents an integer of from1 to 2; L represents a methine linkage, e.g., -OH=, C(CH --C(C H etc.; RR and R each represents an alkyl group, including substituted alkyl(preferably a lower alkyl containing from 1 to 4 carbon atoms), e.g.,methyl, ethyl, propyl, isopropyl, butyl, hexyl, cyclohexyl, decyl,dodecyl, etc., and substituted alkyl groups (preferably a substitutedlower alkyl containing from 1 to 4 carbon atoms), such as a hydroxyalkylgroup, e.g., fl-hydroxyethyl, w-hydroxybutyl, etc., an alkoxyalkylgroup, e.g., p-methoxyethyl, w-butoxybutyl, etc., a carboxyalkyl group,e.g., fi-carboxyethyl, w-carboxybutyl, etc., a sulfoalkyl group, e.g.,B- sulfoethyl, w-sulfobutyl, etc., a sulfatoalkyl group, e.g.,fl-sulfatoethyl, w-sulfatobutyl, etc., an acyloxyalkyl group, e.g.,fl-acetoxyethyl, -acetoxypropyl, w-butyryloxybutyl, etc., analkoxycarbonylalkyl group, e.g., p-methoxycarbonylethyl,w-ethoxycarbonylbutyl, w-ethoxycarbonylbutyl, etc., or an aryl group,e.g., phenyl, tolyl, naphthyl, methoxyphenyl, chlorophenyl, etc.; R andR each represents the same or a different alkyl group containing from 1to 6 carbon atoms, e.g., methyl, ethyl, isopropyl, butyl, 2-cyanoethyl,hexyl, etc.; X and X each represents any acid anion, such as chloride,perchlorate, sulfonate, ptoluenesulfonate and which may be combined in Ror R when the dye is a betaine; Z Z and 2;, each represents thenon-metallic atoms necessary to complete the same or different 5- to6-membered heterocyclic nucleus of the type used in methine dyes, whichnucleus may contain a second hetero atom such as oxygen, sulfur,selenium or nitrogen, such as the following nuclei: a thiazole nucleus,e.g., thiazole, 4-methylthiazole, 4-phenylthiazole, S-rnethylthiazole,S-phenylthiazole, 4,5-dimethylthiazole, 4,5-diphenylthiazole,4-(2-thienyl)thiazole, benzothiazole, 4- chlorobenzothiazole, 5chlorobenzothiazole, 6 chlorobenzothiazole, 7 chlorobenzothiazole, 4methylbenzothiazole, S-methylbenzothiazole, 6-methylbenzothiazole, 5-bromobenzothiazole, 6 bromobenzothiazole, 5 phenylbenzothiazole,4-methoxybenzothiazole, S-methoxybenzothiazole, 6-methoxybenzothiazole,S-iodobenzothiazole, 6- iodobenzothiazole, 4-ethoxybenozthiazole,S-ethoxybenzothiazole, tetrahydrobenzothiazole,5,6-dimethoxybenzothiazole, 5,6 dioxymethylenebenzothiazole, 5hydroxybenzothiazole, 6-hydroxybenzothiazole, naphtho[2,l-d] thiazole,naphtho[ 1,2-d] thiazole, S-methoxynaphtho [2,3- dJthiazole,S-ethoxynaphtho[2,3-d1thiazole, 8-methoxynaphtho[2,3 d]thiazole, 7methoxynaphtho[2,3 d] thiazole,4'-methoxythianaphtheno-7',6',4,5-thiazole, etc.; an oxazole nucleus,e.g., 4-methloxazole, S-methyloxazole,

4-phenyloxazole, 4,5-diphenyloxazole, 4-ethyloxazole, 4,5-dimethyloxazole, S-phenyloxazole, benzoxazole, S-chlorobenzoxazole,S-methylbenzoxazole, S-phenylbenzoxazole, 6-methylbenzoxazole,5,6-dimethylbenzoxazole, 4,6-dimethylbenzoxazole, 5 -methoxybenzoxazole,5 -ethoxybenzoxazole, S-chlorobenzoxazole, 6-methoxybenzoxazole, 5hydroxybenzoxazole, 6 hydroxybenzoxazole, naphtho[2,1-d]0xazole,naphtho[l,2-d]oxazole, etc.; a selenazole nucleus, e.g.,4-methylselenazole, 4-phenylselenazole, benzoselenazole,S-chlorobenzoselenazole, 5- methoxybenzoselenazole,S-hydroxybenzoselenazole, tetrahydrobenzoselenazole,naphtho[2,1-d]selenazole, naphtho,[1,2-d]selenazole, etc.; and aquinoline nucleus, e.g., 2-quinoline, 3-methyl-2-quinoline,S-ethyl-Z-quinoline, 6- chloro 2 quinoline, 8 chloro-2-quinoline,6-methoxy-2- quinoline, 8-ethoxy-2-quinoline, 8-hydroxy-2-quinoline,4-quinoline, 6-methoxy-4-quinoline, 7-methyl-4-quinoline, 8 chloro4-quinoline, l-isoquinoline, 3,4-dihydro-1-isoquinoline, 3-isoquinoline,etc. Dyes of Formula 1 above wherein h represents 1 and Z and Z eachrepresents a quinoline nucleus, i.e., quinoline monomethine cyaninedyes, provide particularly good results in the practice of thisinvention. Some specific dyes which can be used in this invention arelisted below:

2- (p-dimethylaminostyryl)-1-methyl pyridinium salt1,1-diethyl-2,2-cyanine salt 1,1'-dimethyl-2,2'-cyanine salt1,1-dimethyl-2,4'-cyanine salt 1-methyl-1',9-diethyl-2,2'-cyanine salt1,1',3,3-tetramethyl-2,2'-cyanine salt 3,3'-diethylthiacarbocyanine salt3,3-diethyloxacarbocyanine salt l,3-diethylselena-2'-cyanine salt1,1-diphenyl-2,4'-cyanine saltAnhydro-1'-ethyl-3-fi-sulfoethylthia-2'-cyanine hydroxide Dyesparticularly useful in carrying out the method of the present inventionfor detecting infrared radiation of wavelengths from about 2.0 to about3.5 microns are 1,l'-diethyl-2,2'-cyanine chloride,1,1',3,3-tetramethyl-2,2'-cyanine p-toluenesulfonate,3,3'-diethyl-9-methylthiacarbocyanine bromide,3,3-diethylthiacarbocyanine p-toluenesulfonate,l,l'-diethyl-2,4'-cyanine chloride, and 1,1-dimethyl-2,2-cyaninep-toluenesulfonate.

The useful concentration of the dyes which can be employed in thepractice of this invention will be largely dependent upon the type ofdye used, the size and type of the silver halide grains employed, andthe particular results desired. Generally good results are obtained withabout .1 to 1 gram of dye per mole of silver, although higherconcentrations can be employed. With some particular dyes, advantageouseffects are obtained with as low as about 10% of monomolecular layercoverage, with effects increasing with increased dye concentrations upto monomolecular layer coverage. Excess dye over monomolecular layersilver halide coverage can be used, but provides no advantage in thepractice of the invention. It is preferred to employ dyes at aconcentration which will give monomolecular layer coverage, or nearlymonolayer coverage, of the silver halide grains. Typical dyeconcentrations used to spectrally sensitize photographic emulsions fornormal temperature exposures are about .02 to .08 gram dye per mole ofsilver. Such coneentrations do not effectively increase sensitivity andcontrast at temperatures of '180 C. and below. Combinations of dyes canalso be used.

The invention can be practiced with any of the light, sensitivephotographic silver halides, including silvertional negative,developing-out, unfogged silver halide emulsions are used in thisinvention. Advantageously, the average silver halide grain size can beup to .8 micron and preferably is up to about .5 micron with the verybest results being obtained with small cubic grains, the edges of whichare on the order of about .2 micron. Silver bromoiodide and silverbromide emulsions are preferred.

In order to obtain the desired results of this invention, it isadvantageous to employ silver halide grains which are free fromchemicalsensitizers, such as sulfur sensitiziers or noble metal saltsensitizers, and chemical foggants.

The silver halide grains employed in this invention can be coated on anysuitable support, such as paper, cellulose acetate orpoly(ethylene-terephthalate) supports, and the grains can be dispersedin suitable binders, such as gelatin. If desired, the silver halidegrains can be vacuum deposited on a suitable support.

The silver halide emulsions used herein can contain antifoggants,stabilizers, hardeners and plasticizers described and referred to inBeavers US. Pat. 3,039,873 (1962) col. 10-13.

The temperatures used in the practice of the invention are from about-180 C. and below, such as down to the temperature of liquid helium,i.e., about -269 C. Liquid air (-l86 C.) or liquid nitrogen ('--196 C.)can be conveniently used to lower the temperature of silver halide forthe low temperature exposure.

Emulsions which have been given low temperature exposures in accordancewith the invention are advantageously processed under conventionalconditions, i.e., using standard developers and normal temperatures,such as above about C., for example, up to about 50 C., and preferablyat about room temperature or about 20' to 25 C.

The following examples are included for a further understanding of theinvention.

EXAMPLE 1 A gelatin silver iodobromide emulsion is preparedsubstantially as described in French Pat. 1,497,202 without chemicalsensitization. The crystal habit of the silver halide grains in thispreparation is cubic (the edges being about .2 micron) and the emulsionis coated on a cellulose acetate base with 100 mg. Ag/sq. ft. 200 mg.gelatin/ sq. ft. and 800 mg./mole Ag of l,l-diethyl-2,2-cyaninechloride. One 35 mm. strip of this photographic film is exposed for 1minute, while immersed in liquid nitrogen (77 K.), to a 200 watttungsten lamp filtered by one Wratten 36 filter, one Wratten 38A filter(this filter combination limits the exposure to the 370-460 nm. spectralregion). While still immersed in liquid nitrogen a small portion of thisexposed film is then irradiated for 2 minutes with a tungsten sourcefiltered with a germanium filter which limited the exposure towavelengths longer than 1.7 The energy hitting the film is 120 mw./sq.cm. The exposed film is then developed for 12 minutes at 20 C. in asolution of N-methyl-p-aminophenol sulfate 2.5 g., ascorbic acid 10.0g., sodium metaborate 35.0 g., potassium bromide 1.0 g., and water tomake 1 liter. It is then fixed for 3 minutes, washed and dried.Densities are read on a densitometer through a red filter. The measureddensity in the area not irradiated with the radiation having wavelengthslonger than 1.7a is .97. The density in the area irradiated with theradiation with wavelengths greater than 1.7;1. is .06. Percentdestruction of the latent image is 94. Generally similar results areobtained when the 1,l'-diethyl-2,2-cyanine chloride is replaced withsimilar quantities of 1,1',3,3' tetramethyl-2,2.-cyanine pts,3,3-diethyl-9 methylthiacarbocyanine bromide,3,3'-diethylthiacarbocyanine pts, l,l-diethyl-2,4-cyanine chloride and1,l-dirnethyl-2,2'- cyanine pts (pts refers to p-toluenesulfonate).

EXAMPLE 2 The photographic film described in Example 1 is used for theexposure described in Example 2. A strip of this photographic film isexposed for 40 seconds while immersed in liquid nitrogen (77 K.), to a200 Watt tungsten lamp filtered by one Wratten 36 filter, one Wratten38A filter. While still immersed in liquid nitrogen a small portion ofthis exposed film is then irradiated for 20 minutes with a tungstensource filtered with a germanium filter and a Kodak InfraredInterference filter series 420. This filter combination limits theexposure to wavelengths longer than 2.2a. The energy hitting the film is30 mw./ sq. cm. The exposed film is then developed, fixed, washed anddried as in Example 1. The measured density in the area not irradiatedwith the radiation having wavelengths longer than 2.2a is 1.00. Thedensity in the area irradiated with the radiation with wavelengthsgreater than 2.2 is .34. Percent destruction of the latent image is 66.

EXAMPLE 3 The photographic film described in Example 1 is used for theexposure described in Example 3. A strip of this photographic film isexposed for 20 seconds while immersed in liquid nitrogen (77 K.), to a200 watt tungsten lamp filtered by one Wratten 36 filter and one Wratten38A filter. While still immersed in liquid nitrogen a small portion ofthis exposed film is then irradiated for 40 minutes with a neon lasertuned to a wavelength of 3.3914. A 2.2a cuton filter is used to insurethat no shorter wavelength can reach the film. The energy hitting thefilm is 31 mw./cm. The exposed film is then developed, fixed, washed anddried as in Example 1. The measured density in the area not irradiatedwith the 3.39 radiation is 1.10. The density in the area irradiated withthe 339 radiation is 1.00. Percent destruction of the latent image is 9.

The invention has been described in detail with particular reference topreferred embodiments thereof, but, it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

We claim:

1. A photographic process for increasing the infrared sensitivity ofsilver halide grains maintained at a temperature below about -180 C.,said grains having adsorbed thereto a methine sensitizing dye, thehighest occupied electronic energy level of said dye, in itsunaggregated form at room temperature, being more positive than thehighest occupied energy level in the valence band of the silver halide,and the lowest vacant electronic energy level of said dye, in itsunaggregated form at room temperature, being more positive than theconduction band of the silver halide, said dye trapping electrons at adepth of not more than .7 ev. below the conduction band of the silverhalide at temperatures below about 180 C., the silver halide grainshaving a sufficient amount of th dye adsorbed thereto to efiectivelyincrease the speed of the silver halide when exposed at a temperaturebelow about -180 C., which comprises:

(1) cooling the photographic silver halide emulsion to a temperaturebelow about 180 C.; and,

(2) uniformly exposing the cooled emulsion to blue light of a Wavelengthof from about 370 to about 460 nm. forlsufficient time to produce alatent image having a developable density.

2. A photographic process for increasing the infrared sensitivity of aphotographic silver halide emulsion maintained at a temperature belowabout --l80 C., said emulsion comprising silver halide grains havingadsorbed thereto a methine dye, the highest occupied electronic energylevel of said dye, in its unaggregated form at room temperature, beingmore positive than the highest occupied energy level in the valence bandof the silver halide, and the lowest vacant electronic energy level ofsaid dye, in its unaggregated form at room temperature,

being more positive than the highest occupied energy level in thevalence band of the silver halide, and the lowest vacant electronicenergy level of said dye being more positive than the conduction band ofthe silver halide, said dye trapping electrons at a depth of not morethan .7 ev. below the conduction band of the silver halide attemperatures below about 180 C., said dye having at least one of thefollowing formulas:

wherein h, m, n and p each represents an integer of from 1 to 2; Lrepresents a methine linkage; R R and R each represents a memberselected from the group consisting of an alkyl group and an aryl group;R and R each represents an alkyl group containing from 1 to 6 carbonatoms; X and X each represents an acid anion; and, Z Z and Z eachrepresents the atoms necessary to complete a 5- to 6-memberedheterocyclic nucleus of the type used in methine dyes, the silver halidegrains having a sufiicient amount of the dye adsorbed thereto toeffectively increase the speed of the silver halide when exposed at atemperature below about 180 C., which comprises:

(1) cooling the photographic silver halide emulsion to a temperaturebelow about -l80 C.; and

(2) uniformly exposing the cooled emulsion to blue light of a wavelengthof from about 370 to about 460 nm. for sufiicient time to produce alatent image having a developable density.

3. The process as defined in claim 2 wherein said Z Z and Z in thestructural formulas each represents the atoms required to complete anucleus selected from the group consisting of a thiazole nucleus, anoxazole nucleus, a selenazole nucleus, and a quinoline nucleus.

4. The process as defined in claim 2 wherein said dye is adsorbed ontosaid silver halide in a concentration sufficient to give monomolecularlayer coverage of the silver halide grains.

5. A photographic process for rendering a light-sensitive photographicsilver halide emulsion maintained at a temperature below about 180 C.sensitive to infrared radiation of a wavelength from about 2.0 to about3.5 microns, said emulsion comprising silver halide grains havingadsorbed thereto a methine sensitizing dye, said dye selected from thegroup consisting of 1,l-diethyl-2,2-cyanine dye salt;1,l',3,3'-tetramethyl-2'-cyanine dye salt;3,3-diethyl-9-methylthiacarbocyanine dye salt;3,3'-diethylthiacarbocyanine dye salt; 1,l'-diethyl-2,4'-cyanine dyesalt; and l,1-dimethyl-2,2'-cyanine dye salt which comprises:

(1) cooling the photographic silver halide emulsion to a temperaturebelow about l80 C.; and (2) uniformly exposing the cooled emulsion toblue light of a wavelength of from about 370 to about 460 nm. forsufiicient time to produce a latent image having a developable densityof about 1.0.

6. A photographic process for rendering a light-sensitive photographicsilver halide emulsion maintained at a temperature below about 180 C.sensitive to infrared radiation of a wavelength from about 2.0 to about3.5 microns, said emulsion comprising an unfogged cubic regular-grainsilver bromoiodide gelatin emulsion, the grains of said emulsion havinga size of about 0.2 micron, said grains having adsorbed thereto amonomolecular layer coverage of a 1,1'-diethyl-2,2'-cyanine dye salt,which comprises:

(1) cooling the photographic silver halide emulsion to a temperaturebelow about C.; and

(2) uniformly exposing the cooled emulsion to blue light of a wavelengthof from about 370 to about 460 nm. for sufficient time to produce alatent image having a developable density of at least about 1.0.

7. In a photographic process in which light-sensitive silver halidegrains are cooled to a temperature below about 180 C., uniformly exposedto radiation having a wavelength of about 370 to about 460 nm. toproduce a developable latent image and, while maintaining thetemperature of the grains below about 180 C., giving said grains animagewise Herschel exposure to infrared radiation to at least partiallydestroy said latent image, said silver halide grains being essentiallyfree from chemical sensitizers and having an average grain size up toabout .8 micron, the improvement which comprises: employing in saidprocess silver halide grains Which have adsorbed thereto a methinesensitizing dye, the highest occupied electronic energy level of saiddye, in its unaggregated form at room temperature, being more positivethan the highest occupied energy level in the valence band of the silverhalide, and the lowest vacant electronic energy level of said dye being,in its unaggregated form at room temperature, more positive than theconduction band of the silver halide, said dye trapping electrons at adepth of not more than .7 ev. below the conduction band of the silverhalide at temperatures below about 180 C., said silver halide grainshaving a sufiicient amount of said dye adsorbed thereto to etfectivelyincrease the speed of said silver halide when exposed at a temperaturebelow about 180 C.

8. In a photographic process in which light-sensitive silver halidegrains are cooled to a temperature below about 180 C. uniformly exposedto radiation having a wavelength of about 370 to 460 nm. to produce alatent image developable to a density of at least 1.0 and, Whilemaintaining the temperature of the grains below about 180 C., givingsaid grains a Herschel exposure to infrared radiation to at leastpartially destroy said latent image, said silver halide grains beingfree from chemical sensitizers, and the halide of said silver halidebromide or bromoiodide, said grains having an average grain size up toabout .5 micron, the improvement which comprises: employing in saidprocess silver halide grains which have adsorbed thereto a methinesensitizing dye, the highest occupied electronic energy level of saiddye being more positive than the highest occupied energy level in thevalence band of the silver halide, and the lowest vacant electronicenergy level of said dye being, in its unaggregated form at roomtemperature, more positive than the conduction band of the silverhalide, said dye trapping electrons at a depth of not more than .7 ev.below the conduction band of the silver halide at temperatures belowabout 180 C., said dye having at least one of the following formulas:

wherein h, m, n and I each represents an integer of from 1 to 2; Lrepresents a methine linkage; R R and R each represents a memberselected from the group consisting of an alkyl group and an aryl group;R and R each represents an alkyl group containing from 1 to 6 carbonatoms; X and X each represents an acid anion; and, Z Z and Z eachrepresents the atoms necessary to complete a 5- to G-memberedheterocyclic nucleus of the type used in methine dyes, said silverhalide grains having a sufficient amount of said dye adsorbed thereto toeffectively increase the speed of said silver halide when exposed at atemperature below about --180 C.

References Cited Blair, A Note on Photographic Sensitivity Effect JOSA,vol. 24, pp. 135-156, June 1934.

James et al., Photographic Science and Engineering, vol. 4, No. 4(1963).

NORMAN G. TORCHIN, Primary Examiner 5 R. FICHTER, Assistant Examiner US.Cl. X.R.

